This is a randomized, two-arm, open-label, multicenter, study comparing the safety and efficacy of Iodine-131 Anti-B1 Antibody to unlabeled Anti-B1 Antibody for the treatment of chemotherapy-refractory low-grade B-cell non-Hodgkin's lymphoma (NHL). Seventy-eight patients will be randomized to receive iodine-131 Anti-B1 Antibody (Arm A) or unlabeled Anti-B1 Antibody (Arm B). Patients randomized to receive unlabeled Anti-B1 Antibody may crossover and receive radiolabeled Iodine-131 Anti-B1 Antibody following progression of their lymphoma. Response in both arms will be assessed at 6 weeks, 3 months, and then at 3-monthly intervals for two years.